regmap: regmap: avoid spurious warning in regmap_read_debugfs

Gcc warns about the case where regmap_read_debugfs tries to walk an
empty map->debugfs_off_cache list, which would results in uninitialized
variable getting returned, if we hadn't checked the same condition
just before that.

After an originally suggested inferior patch from Arnd Bergmann,
this is the solution that Russell King came up with, sidestepping
the problem by merging the error case for an empty list with the
normal path.

Without this patch, building mxs_defconfig results in:

drivers/base/regmap/regmap-debugfs.c: In function 'regmap_read_debugfs':
drivers/base/regmap/regmap-debugfs.c:147:9: : warning: 'ret' may be used uninitialized in this function [-Wmaybe-uninitialized]

Reported-by: Vincent Stehle <v-stehle@ti.com>
Cc: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>